If the meter has the rectifiers built in, then a series dropping resistor might be enough to get you going. If you needed a few different ranges, it could be a switch and a resistor.

Just for easy numbers, assume 0 VU on the meter was set up for 1 V rms.
A 100 Watt amp puts out about 28 Vrms max. If you wanted the 28 volts to correspond to 0 VU, put a resistor of 27*1200=32400 in series.

The meter, with such a resistor would probably never move until the volume was really high...so, that's where you might want a range switch...

you could even use "decade engineering". Throw a decade resistor box in series with the meter, all of which is hooked across the speaker terminals...adjust the R downard from 33K until you have the desired sensitivity.